In quantum computing, a qubit or quantum bit (sometimes qbit) is the basic unit of quantum information: 4

4 (four) is a number, numeral and digit. It is the natural number following 3 and preceding 5. It is the smallest composite number, and is considered unlucky in many East Asian cultures.

Four is the smallest composite number, its proper divisors being 1 and 2.

4 is the smallest squared prime (p2) and the only even number in this form. 4 is also the only square one more than a prime number.

A number is a multiple of 4 if its last two digits are a multiple of 4. For example, 1092 is a multiple of 4 because 92 = 4 × 23.

Brahmic numerals represented 1, 2 and 3 with as many lines. 4 was simplified by joining its four lines into a cross that looks like the modern plus sign. The Shunga would add a horizontal line on top of the digit, and the Kshatrapa and Pallava evolved the digit to a point where the speed of writing was a secondary concern. The Arabs’ 4 still had the early concept of the cross, but for the sake of efficiency, was made in one stroke by connecting the “western” end to the “northern” end; the “eastern” end was finished off with a curve. The Europeans dropped the finishing curve and gradually made the digit less cursive, ending up with a digit very close to the original Brahmin cross.

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